Biology: Reproduction

Question 1

Karokinesis

Answer 1

Nuclear division.

Question 2

Centromere

Answer 2

Central region that holds together sister chromatids.

Question 3

Chromatin

Answer 3

Uncoiled DNA during interphase.

Question 4

Prophase

Answer 4

The chromosomes condense, and the centriole pairs (in animals) separate and move toward the opposite poles of the cell. The spindle apparatus forms between them, and the nuclear membrane dissolves, allowing the spindle fibers to interact with the chromosomes.

Question 5

Metaphase

Answer 5

The centriole pairs are now at opposite poles of the cell. The fibers of the spindle apparatus attach to each chromatid at the centromere to align the chromosomes at the equator of the cell, forming the metaphase plate.

Question 6

Anaphase

Answer 6

The centromeres split so that each chromatid has its own distinct centromere, thus allowing sister chromatids to separate. The sister chromatids are pulled toward the opposite poles of the cells by the shortening of the spindle fibers. Spindle fibers are composed of microtubules.

Question 7

Telophase

Answer 7

The spindle apparatus disappears. A nuclear membrane forms around each set of newly formed chromosomes. Thus, each nucleus contains the same number of chromosomes (the diploid number 2N) as the original or parent nucleus. The chromosomes uncoil, resuming their interphase form.

Question 8

Cytokinesis

Answer 8

Near the end of telophase, the cytoplasm divides into two daughter cells, each with a complete nucleus and its own set of organelles. In animal cells, a cleveage furrow forms, and the cell membrane indents along the equator of the cell and finally pinches through the cell, separating the two nuclei.

Question 9

Prophase I

Answer 9

The chromatin condenses into chromosomes, the spindle apparatus forms, and the nucleoli and nuclear membrane disappear. Homologous chromosomes (chromosomes that code for the same traits, one inherited from each parent) come together and intertwine in a process called synapsis. Since each chromosome has 2 sister chromatids, each synaptic pair of homologous chromosomes contains four chromatids and is therefore often called a tetrad. Sometimes chromatids of homologous chromosomes break at corresponding points and exchange equivalent pieces of DNA; this process is called crossing over. Note that crossing over occurs between homologous chromosomes and not sister chomatids of same chromosomes. The chromatids involved are left with an altered but complete set of genes. Recombination among chromosomes results in diversity.

Question 10

Metaphase I

Answer 10

Homologous pairs (tetrads) align at the equatorial plane, and each pair attaches to a separate spindle fiber by its kinectochore.

Question 11

Anaphase I

Answer 11

The homologous pairs separate and are pulled to opposite poles of the cell. This process is called disjunction, and it accounts for a fundamental Mendelian law. During disjunction, each chromosome of paternal origin separates from its homologue of maternal origin, and either chromosome can end up in either daughter cell. Thus, the distribution of homologous chromosomes to the 2 intermediate daughter cells is random with respect to parental origin. Each daughter cell gets a unique pool of genes.

Question 12

Telophase I

Answer 12

A nuclear membrane forms around each new nucleus. At this point, each chromosome still consists of sister chromatids joined at the centromere.

Question 13

Second Meiotic Division

Answer 13

Very similar to mitosis, except Meiosis II not preceded by chromosomal replication. The new cells have the haploid number of chromosomes.

Question 14

Gonads

Answer 14

Organs that produce gametes. Male ones called testes, which produce sperm in tightly coined seminiferous tubules. Female ones called ovaries produce oocytes (eggs).

Question 15

Internal Fertilization

Answer 15

Practiced by terrestrial vertebrates and provides a direct route for sperm to each the egg cell. This increases the chance for fertilization success, and females produce fewer eggs. The number of eggs produced is affected by other factors as well. If the early development of the offspring is outside the mother’s body, more eggs will be laid to increase chances of offspring survival. The amount of parental care after is also related to the amount of eggs produced. Species that care for their young produce fewer eggs.

Question 16

Male Reproductive Physiology

Answer 16

The testes are in an external pouch called the scrotum that maintains the testes’ temperature at 2-4 degrees Celsius lower than the body temperature, a condition essential for sperm survival. Sperm pass from the testes through the vas deferens to the ejaculatory duct and then to the urethra. The urethra passes through the penis and opens to the outside at its tip. In males, urethra’s a common passageway for both reproductive and excretory systems.

Question 17

Spermatogenesis

Answer 17

Sperm production. Occurs in the seminiferous tubules.

Diploid cells called spermatogonia differentiate into diploid cells called primary spermatocytes, which undergo the first meiotic division to yield 2 haploid secondary spermatocytes of equal size; the 2nd meiotic division yields 4 haploid spermatids of equal size. After meiosis, the spermatids undergo a series of changes leading to the production of mature sperm, or spermatozoa, which are specialized for transporting the sperm nucleus to the egg, or ovum.

After a male has reached sexual maturity, approximately 3 million primary spermatocytes begin spermatogeneis a day, maturation process taking 65-75 days.

Question 18

Female Reproductive Anatomy

Answer 18

Ovaries found in abdominal cavity, below digestive system. They consist of thousands of follicles, which is a multilayed sac of cells that has, nourishes, and protects an immature ovum. The follicle cells produce estrogen.

Once a month, an immature ovum is released from ovary into the abdominal cavity drawn into the nearby oviduct, also known as the fallopian tube. Each tube opens into the upper end of a muscular chamber called the uterus, which is the site of fetal development. The lower, narrow end of the uterus is called the cervix. It connects with the vaginal canal, the site of sperm deposition during intercourse and also the path through which a baby is expelled during child birth.

At birth, all the eggs that a female will ovulate during her lifetime are already present in the ovaries.

Question 19

Oogenesis

Answer 19

Production of female gametes in the ovarian follicles. At birth, all of the immature ova (primary oocytes) that a female makes during her lifetime are already in her ovaries. Primary oocytes are diploid cells that form by mitosis in ovary. After menarche (the first time a female gets her period), one primary oocyte per month completes meiosis I, yielding 2 daughter cells of unequal size–a secondary oocyte and a small cell known as a polar body. The secondary oocyte is expelled from follicle during ovulation. Meiosis II doesn’t occur until fertilization.

The oocyte cell membrane is surrounded by 2 layers of cells; the inner layer is the zona pellucida, and the outer layer is the corona radiata. Meiosis II occurs when these layers are penetrated by sperm, yielding 2 haploid cells–a mature ovum and another polar body. (1st polar body may also undergo Meiosis II, but eventually both polar bodies will die). The mature ovum is a large cell with alot of cytoplasm, RNA, organelles, and nutrients needed by a developing embryo.

Question 20

Estrogens

Answer 20

Steroid hormones necessary for normal female maturation. They stimulate the development of the female reproductive tract and contribute to the development of secondary sexual characteristics and sex drive. They’re also responsible for the thickening of the endometrium (uterine wall). Estrogens are secreted by the ovarian follicles and the corpus luteum.

Question 21

Progesterone

Answer 21

A steroid hormone secreted by the corpus luteum during the luteal phase of the menstrual cycle. It stimulates development and maintenance of the endometrial walls in preparation for implantation.

Question 22

The Menstrual Cycle

Answer 22

The hormonal secretions of the ovaries, hypothalmus, and anterior pituitary play important roles in female reproductive cycle. From puberty through menopause, interactions between these hormones result in the monthly menstrual cycle. It can be divided into the follicular phase, ovulation, the luteal phase, and menstruation.

Question 23

Follicular Phase

Answer 23

Beings with the cessation of the menstrual flow from the previous cycle. During the phase, FSH (follicle stimulating hormone) from the anterior pituitary promotes the development of the follicle, which grows and begins secreting estrogen.

Question 24

Ovulation

Answer 24

Midway through the cycle, ovulation occurs. A mature ovarian follicle bursts and releases an ovum. Ovulation is caused by a surge in LH (Lutenizing hormone) that’s preceded, and in part caused, by a peak in estrogen levels.

Question 25

Luteal Phase

Answer 25

After ovulation, LH induces the ruptured follicle to develop into the corpus luteum, which secretes estrogen and progesterone. Progesterone causes the glands of the endometrium to mature and produce secretions that prepare it for the implantation of an embryo. Progesterone and estrogen are essential for the maintenance of the endometrium.

Question 26

Menstruation

Answer 26

If the ovum is not fertilized, the corpus luteum atrophies. The resulting drop in progesterone and estrogen levels causes the endometrium (with its superficial blood vessels) to slough off, giving rise to the menstrual flow (menses).

If fertilization occurs, the developing placenta produces hCG (human chorionic gonadtrophin), maintaining the corpus luteum and, thus, the supply of estrogen and progesterone that maintains the uterus, until the placenta takes over production of these hormones.

Question 27

Fertilization

Answer 27

An egg can be fertilized during the 12-24 hours of ovulation. Fertilization occurs in the lateral widest portion of the fallopian tube. Sperm must travel through the vaginal canel, cervix, unterus, and into the fallopian tubes to reach the ovum. Sperm remain viable and capable of fertilization for 1-2 days after intercourse.

The first barrier that the sperm must penetrate is the corona radiata. Enzymes secreted by the sperm aid in penetration of the corona radiata. The acrosome is responsible for penetrating the zona pellucida; it releases enzymes that digest this layer, thereby allowing the sperm to come into direct contact with the ovum cell membrane. Once in contact with the membrane it penetrates it, fusing the sperm cell membrane with that of the ovum. The sperm nucleus now enters the ovum’s cytoplasm. It is at this stage of fertilization that the ovum completes meiosis II.

The acrosomal reaction triggers a cortical reaction in the ovum, causing calcium ions to be released into the cytoplasm; this, in turn, initiates a series of reactions that result in the formation of the fertilization membrane. The fertilization membrane is a hard layer that surrounds the ovum cell membrane and prevents multiple fertilizations. The release of calcium also stimulates metabolic changes within the ovum, greatly increasing its metabolic rate. This is followed by the fusion of the sperm nucleus with the ovum nucleus to form a diploid zygote. The first mitotic division of the zygote soon follows.

Question 28

Monozygotic Twins

Answer 28

Result when a single zygote splits into two embryos. If the splitting occurs at the two-cell stage of development, the embryos will have separate chorions and separate placentas; if it occurs at the blastula stage, then the embryos will have only one chorionic sac and will therefore share a placenta and possibly an amnion. Occasionally, the division is incomplete, resulting in “Siamese” (conjoined) twins, which are attacked at some point on the body, often sharing limbs or organs. Monozygotic twins are genetically identical because they develop from the same zygote. Monozygotic twins are therefore of the same sex, blood type, and so on.

Question 29

Dizygotic Twins

Answer 29

Result when two ova are released in one ovarian cycle and are fertilized by two different sperm. The two embryos implant in the uterine wall individually, and each develops its own placenta, amnion, and chorion (although the placentas may fuse if the embryos implant very close to each other). Fraternal twins share no more characteristics than any other siblings because they develop from two distinct zygotes.

Question 30

Hermaphrodites

Answer 30

Some species are hermaphrodites, which have both functional male and female gonads.

Question 31

Testosterone

Answer 31

Testes also make testosterone, which regulates secondary male sex characteristics including facial and public hair and voice changes.

Question 32

Sperm Structure

Answer 32

The mature sperm is an elongated cell with a head, neck, body, and tail. The head consists almost entirely of nucleus. The tail (flagellum) propels the sperm, while mitochondria in the neck and body provide energy for locomotion. A caplike structure called the acrosome, derived from Golgi apparatus, develops over the anterior half of the head. The acrosome contains enzymes needed to penetrate the tough outer covering of the ovum.

Question 33

Menopause

Answer 33

Woman ovulate once every 4 weeks (except during pregnancy and, usually, lactation) until menopause, which typically occurs age 45-50. During it, the ovaries become less sensitive to the hormones that stimulate follicle development (FSH and LH) and eventually they atrophy (waste away). The remaining follicles disappear, estrogen and progesterone levels greatly decline, and ovulation stops. The profound changes in hormone levels are often accompanied by physiological and psychological changes that persist until a new balance is reached.